1. Field of the Invention
The present invention relates to an engine operation control device for controlling an engine as it shifts its operating state from a non-idling state to an idling state.
2. Description of the Prior Art
In recent years a variety of electronic control fuel injection systems for diesel engines have been developed which can control a fuel injection pressure as well as a fuel injection amount and a fuel injection timing in order to make further improvements in the engine characteristics involving output and mileage and in the exhaust gas characteristics. Such fuel injection systems for engines have an injector that includes a needle valve, which moves up or down in an injector body to perform an open-close control on injection nozzle holes, and a solenoid valve, which is supplied with a drive current for controlling a working fluid to raise or lower the needle valve. According to an operating condition of the engine, the timing, amount and pressure of the fuel injected from the injector are controlled by a controller.
Among such electronic control fuel injection systems proposed so far are a hydraulically activated system and a fuel pressure activated system. In the hydraulically activated system, an engine oil is used as the working fluid that is pressurized by a high pressure oil pump, the injector has a pressure increasing piston therein which operates on the pressure of the engine oil, and the fuel in a pressure increasing chamber is pressurized by the pressure increasing piston to lift the needle valve, which in turn allows the pressurized fuel to be injected from nozzle holes opened by the needle valve. In the fuel pressure activated system, a high-pressure fuel is used as the working fluid that is pressurized by a high-pressure fuel pump and stored in a common rail, the injector has a pressure control chamber formed in its body and controls the inflow and outflow of the high-pressure fuel into and out of the pressure control chamber to lift or lower the needle valve according to the pressure of the high-pressure fuel, thereby injecting the high-pressure fuel from the nozzle holes opened by the needle valve. In either type of the electronic control fuel injection system, the injector has a solenoid valve, and a controller in the form of an electronic control device controls the timing and duration of supplying a drive current to the solenoid valve to supply the highly pressurized working fluid to the injector, which in turn injects fuel in a predetermined amount at a predetermined timing from nozzle holes formed at the front end of the injector.
Under the non-idling condition a target fuel injection amount is determined based on data, such as a map which is preset so that the engine output characteristic and exhaust gas characteristic remain optimum in response to the engine revolution speed and load (for example, accelerator opening (or the amount by which the accelerator pedal is depressed)). During idling it is desired that the engine revolution remain constant and thus the target fuel injection amount is determined by setting a target revolution speed for the idling operation and performing a PID control, which is based on a difference between the target revolution speed and the engine revolution speed, so that the engine revolution speed matches the target revolution speed. A decision on whether the engine is idling or not is based, for example, on the engine revolution speed and the accelerator pedal depression amount (accelerator opening). The target revolution speed for idling is determined by correcting a basic revolution speed according to the on/off state of an air conditioner and a warming-up switch, the basic revolution speed being set based on data which was determined beforehand according to the engine temperature (for example, a cooling water temperature detected by an engine cooling water temperature sensor). The target fuel injection amount for idling is determined by adding to a basic fuel injection amount set according to the engine temperature a PID correction amount which is obtained based on the revolution speed difference described above. The target fuel injection amount obtained through the correction is injected during idling to prevent cyclic changes and offsets in revolution speed as well as delays in following rapid revolution speed changes.
As one of the electronic control fuel injection systems that adopt an unit injector of the above hydraulically activated type, there is an electronic control fuel injection system disclosed in Published Japanese translations of PCT international publication No. 511526/1994. In this electronic control fuel injection system the pressure of the engine oil as the working fluid is controlled through an electronic device such as solenoid valve installed in the injector to allow simultaneous control of the fuel injection amount and the fuel injection timing.
An injector 50 shown in FIG. 10 includes a nozzle body 52 having nozzle holes 64 for injecting fuel formed at its front end, a solenoid body 53 mounting a solenoid 60 as a solenoid actuator, an injector body 54 and a fuel supply body 55. The injector 50 has a pressure increasing chamber 57 supplied with a fuel from a common rail 63, a pressure chamber 58 supplied with a working fluid, a pressure increasing piston 59 driven by the working fluid supplied to the pressure chamber 58 to pressurize the fuel in the pressure increasing chamber 57, a return spring 71 for resetting the pressure increasing piston 59, and a case 56 formed with a fuel supply port 61 and a fuel discharge port 62, both opening to the common rail 63 to form a fuel chamber 70. In the injector 50 the needle valve 65 is moved up or down by the fuel pressure from the pressure increasing chamber 57 to open or close nozzle holes 64. The pressure increasing piston 59 comprises a large-diameter portion 68, which is slidably fitted in a hole 66 formed in the injector body and forms part of a wall surface of the pressure chamber 58, and a small-diameter portion 69, which is slidably fitted in a hole 67 and forms part of a wall surface of the pressure increasing chamber 57.
The fuel pressurized by a fuel pump to a relatively low pressure is supplied through the common rail 63, the fuel supply port 61 and the fuel chamber 70 into the pressure increasing chamber 57. The fuel in the pressure increasing chamber 57 is pressurized by the pressure increasing piston 59 and delivered from the pressure increasing chamber 57 at a fuel injection pressure. The engine oil as the working fluid that is pressurized by a high-pressure oil pump to a high pressure is accumulated in a high-pressure oil manifold (or an oil rail, see FIG. 9). To actuate the pressure increasing piston 59, the oil rail is connected to the pressure chamber 58 in the injector 50 and a solenoid valve 51 is installed in a hydraulic pressure passage in the injector 50 through which the engine oil is fed. A drive current from the controller energizes the solenoid 60 to operate a valve disc 72 thus opening the solenoid valve 51, with the result that the engine oil is supplied through the hydraulic pressure passage to the pressure chamber 58, as shown by an arrow, acting on a pressure receiving surface of the pressure increasing piston 59 to drive (or stroke) the pressure increasing piston 59. The fuel in the pressure increasing chamber 57 is pressurized by the pressure increasing piston 59 and as the needle valve 65 is moved up or down in the body of the injector 50 by the fuel pressure from the pressure increasing chamber 57, the nozzle holes 64 formed at the front end of the nozzle body 52 are opened or closed to inject the fuel into the combustion chamber through the open nozzle holes 64. Because the injector 50 pressurizes the fuel in the pressure increasing chamber 57 by the pressure increasing piston 59, the fuel injection is carried out at a fuel injection pressure independent of the engine revolution.
Since the fuel injection pressure is determined by the pressure of the working fluid, or the oil rail pressure, acting on the pressure increasing piston 59, the fuel injection pressure can be controlled by controlling a flow control valve incorporated in the high-pressure oil pump to change the oil rail pressure. The flow control valve uses a solenoid valve whose opening degree is controlled by a duty ratio. By controlling the amount of oil fed from the high-pressure oil pump through the flow control valve to the oil manifold, the oil rail pressure can be controlled. The duty ratio, a control quantity of the flow control valve, is determined according to a target rail pressure, which is obtained by correcting a basic target rail pressure by a PID control that is based on the difference between the basic target rail pressure and the actual rail pressure, the basic target rail pressure being determined by the engine operating condition, namely the engine revolution speed and the target injection amount.
As described above, the electronic control fuel injection system of the hydraulically activated type has a controller which calculates the target injection amount, the target injection timing and the target injection pressure (target rail pressure) according to the operating condition of the engine. Based on the respective target values, the controller determines the current supply duration and timing for the solenoid valve in the injector and the duty ratio of a control current output to the flow control valve in the high-pressure oil pump.
In addition to the electronic control fuel injection system of the hydraulically activated type, there has been known a fuel injection system of fuel pressure activated type in which the injector is operated according to the highly pressurized fuel pressure. This type of fuel injection system is disclosed, for example, in Japanese Patent Publication No. 19381/1992. FIG. 12 is a cross section showing an example of the injector used in the fuel pressure activated fuel injection system. This injector performs fuel injection by supplying a high pressure fuel to a pressure control chamber formed on the back pressure side of the needle valve and leaking the high pressure fuel to control the lift of the needle valve.
In this fuel injection system that uses the highly pressurized fuel as a working fluid, the high pressure fuel is stored in the common rail (see reference number 78 in FIG. 11), from which it is supplied through fuel feed pipes 88 to individual injectors 80. The injectors 80 are each connected to the corresponding fuel feed pipe 88 through a fuel inlet joint 90 provided on the upper side portion of the injector 80. Inside an injector body 81 that forms the injector 80 there are formed fuel passages 91, 92. The fuel feed pipe 88 and the fuel passages 91, 92 together form a fuel path. A part of the fuel supplied from the common rail through the fuel path reaches a fuel reservoir 93 formed in a nozzle 82, from which it is forced through a passage surrounding a needle valve 84 slidable in a hole 83 and is injected into the combustion chamber from nozzle holes 85 that are formed at the front end of the nozzle 82 and opened when the needle valve 84 is lifted. The needle valve 84 has a tapered surface 94, which receives the pressure of the high pressure fuel supplied to the fuel reservoir 93, and is subjected to a force produced by the pressure of the high pressure fuel that urges the valve in the lifting direction. Excess fuel is returned to the common rail through a return pipe 89.
The injector 80 has a needle valve lift mechanism of pressure control chamber type to control the lift of the needle valve 84. That is, the high pressure fuel pressurized by the high-pressure fuel pump, in addition to being injected from the nozzle holes 85, is also supplied to a pressure control chamber 100. The injector 80 has a solenoid valve 96 as a control valve in its head portion, which has a solenoid 98 supplied with a drive current as a control signal from the controller 95 via a signal line 97. When the solenoid 98 is energized, an armature 99 is lifted opening an open-close valve 102 provided at the end of a fuel passage 101 as a leakage path, with the result that the fuel supplied from the fuel path to the pressure control chamber 100 is discharged, releasing the high pressure of the fuel from the pressure control chamber 100 through the oil passage 101.
A control piston 104 is installed vertically movable in a center hole 103 formed in a central part of the body of the injector 80. When the solenoid valve 96 is operated, a force urging the control piston 104 downwardly, which is generated by a combination of the reduced pressure in the pressure control chamber 100 and the spring force of the return spring 105, is overcome by a force urging the control piston 104 upwardly, which is generated by the fuel pressure acting on the tapered surface 94 exposed to the fuel reservoir 93 and on the front end portion of the needle valve 84. Hence, the control piston 104 and therefore the needle valve 84 are lifted, allowing the fuel to be injected from the nozzle holes 85. The amount of fuel injected is determined by the fuel pressure in the fuel path and the lift of the needle valve 84 (the amount and duration of the lift). The drive current supplied to the solenoid 98 is a pulse current to perform an open-close control on the open-close valve 102.
Because the fuel injection pressure is determined by the pressure of the high pressure fuel supplied to the injector 80, the fuel injection pressure can be controlled by controlling the flow control valve installed in the high-pressure fuel pump to change the common rail pressure. As in the hydraulically activated system, the flow control valve uses a solenoid valve that is controlled by the duty ratio. Controlling the duty ratio of a control current applied to the flow control valve enables the common rail fuel pressure to be changed and therefore the fuel injection pressure to be controlled.
As described above, the controller in the fuel pressure activated type electronic control fuel injection system calculates, in the same way as in the hydraulically activated system, the target injection amount, the target injection timing and the target injection pressure (target rail pressure) according to the engine operating conditions and, based on the calculated target values, determines the duration and timing of energizing the solenoid valve in the injector and the duty ratio of a control current output to the flow control valve in the high-pressure fuel pump.
In the above fuel injection system, the injection pressure is set low when the load is small and high when the load is large. This is because a high pressure injection when performed at a low load will increase the pre-mixed combustion ratio, increasing engine noise and NOx in exhaust gas, while on the other hand a low pressure injection when performed at a high load will extend the injection duration deteriorating the mileage and increasing smoke in exhaust gas. Therefore, when the engine operation shifts to idling after the load has increased, the working fluid pressure undergoes a sudden fall after being pressurized to a high pressure. During this rapid pressure reduction air content in the working fluid may appear as bubbles.
If these bubbles should enter into the pressure chamber in the hydraulically activated type injector or into the pressure control chamber in the high pressure fuel type injector, the working fluid pressure in the pressure chamber may not be sufficient to push down the pressure increasing piston or the pressure in the pressure control chamber may fail to be released thoroughly, either case of which will reduce the amount of fuel actually ejected from the injector. As a result, during idling, variations occur in the fuel injection amount among the cylinders or among different cycles, causing unpleasant rotary vibrations of the engine, or what may be termed as swaying vibrations. In the system disclosed in Published Japanese translations of PCT international publication No. 511526/1994, bubbles may also get into oil when the oil returning to the oil pan is agitated by the crankshaft during a high speed operation of the engine.